This invention relates to electrical igniters.
This invention is particularly concerned with methods of forming electrical igniters.
Electrical igniters usually have an outer electrode formed by a tubular metal shell, and an inner electrode formed by a metal rod extending coaxially within the shell. The inner and outer electrodes are separated from one another at an operative tip of the igniter by a small distance so as to define a path for electrical discharge between the electrodes; such path may be across an air-gap or over a semiconductive surface between the two electrodes at the operative tip. The inner electrode is insulated from the outer electrode along its length, the outer electrode being formed with a screw thread or similar formation, for use in mounting the igniter to the engine housing such that it extends through the wall of the combustion chamber with its operative tip located in the zone of combustion. External electrical connection to the two electrodes is made via an electrical connector which engages the ends of the electrodes remote from the operative tip and which is secured to the igniter by a screw-threaded retaining ring that engages with a co-acting thread on the outer electrode. When suitably high electrical energy is applied to the electrodes via the connector, discharge occurs between the electrodes at the operative tip thereby igniting any fuel-air mixture within the combustion chamber.
During manufacture of such igniters it is important that a pressure-tight seal is formed between the two electrodes in order to prevent leakage from the combustion chamber of the engine. In such igniters, typical faults experienced during testing following manufacture, are, an incorrectly functioning spark gap, commonly due to a faulty semiconductor pellet, and leakage through the seals between the electrodes which are particularly susceptible to temperature changes. A proportion of igniters tested show either or both of these faults and must consequently be rejected. This is very expensive, since the entire igniter, which may typically be sixteen centimeters long, must be rejected, it being very difficult to salvage any of its component parts for re-use.
Another disadvantage of the previous form of igniter is that, because of the different configurations of many engine housings, igniters for different engines require igniter bodies having different outer shapes and sizes so as correctly to fit the appropriate engine. This entails a large financial outlay by the manufacturer in order to keep adequate stocks of each of the different configurations of igniters to satisfy any expected demand for igniters for use with different engines. Expensive changes of equipment are also required when manufacturing igniters for different engines.